1. Field of the Invention
This invention relates generally to the field of synthetic liquid hydrocarbon catalysis. More specifically, the invention relates to a catalyst support solution comprising crystalline silica which may be used to form an attrition resistant precipitated Fischer-Tropsch catalyst.
2. Background of Invention
Fischer-Tropsch (FT) synthesis represents a catalytic method for the creation of synthetic liquid fuels. The reaction occurs by the metal catalysis of an exothermic reaction between carbon monoxide and hydrogen gas mixtures, called synthesis gas, or syngas. The liquid product of the reaction is refined to produce a range of synthetic fuels, lubricants and waxes. The primary metals utilized as catalysts are cobalt and iron. The latter is favored due to a significantly lower cost. The quantity of catalyst available for catalysis in the reactor dictates the reaction product synthesized. Large scale Fischer-Tropsch reactors utilize complex systems to maintain nearly static quantities of catalyst within the reactor as a means to produce a constant output of product. Attrition, the degradation of the catalyst structure, is a major hurdle in improving FT reactor efficiency.
Breakage of catalyst structure is mainly attributed to two causes, being physical and chemical attrition. Chemical attrition is associated with structural changes during chemical transformation within the catalyst—typically active phase transition from iron oxide to iron metal to iron carbide. Physical breakup is mainly contributed to the rubbing and collision of the catalyst particles, resulting in micron fine material (fines); very much like rocks in a stream, milling against each other to form edge free pebbles.
The physical integrity of unsupported precipitated iron catalyst suffers during slurry phase Fischer-Tropsch synthesis, degrading product quality (solids and iron content in wax) to such an extent that the run may have to be compromised or terminated. Other impacts may be on the wax upgrading, for example hydrogenation system, which is sensitive to the presence of iron in the feed stock. These negative impacts reduce time online for a reactor and increase costs for filtering product, maintaining the reactor, and overall production.
Several research studies indicate that silica incorporation during, or after precipitation, may lead to catalyst with superior structural properties. However, to create smooth round particles, precipitated catalyst must be prepared by spray drying. Little is published on the subject of catalyst spray drying procedures and conditions. What is available indicates the principle spray drying parameter to successful large, stable round particles, relates to increased particle density. While several spray dryer parameters are associated with the quality of the material spray dried, the main common parameters to establish dense particles are higher solids content in the feed, few soluble species, and slower drying through lower gas temperatures.
Apart from attributing structural integrity, the silica is also thought to assist in creating smooth round catalyst particles that will be subject to minimum attrition. The silica content is however found to have a profound impact on mean particle size, i.e. decreasing as the silica content increases. Hence, the spray drying parameters have to be redefined, and optimized, for each change in catalyst composition. Traditionally silica is added to the catalyst as a structural support. Unsupported catalyst has the tendency to sinter during Fischer-Tropsch synthesis. Reduced iron entities are very mobile, and in the absence of a structural support will coalesce to form bigger entities, which results in a loss of surface area, and consequently leads to a loss in activity.
Consequently, there is a need for a silica support solution which may be used with a precipitated iron catalyst to increase attrition resistance of particles formed therefrom and utilized in a Fischer-Tropsch conversion process.